A CVT typically incorporates a unit referred to as a “variator” having a rotary variator input, a rotary variator output, and a mechanism for transmitting drive from one to the other while enabling the ratio of output speed to input speed (the “variator ratio”) to be steplessly varied.
A typical CVT further comprises gearing by means of which the variator is coupled between an overall transmission input (e.g. the drive shaft of a vehicle engine) and output (e.g. the final drive of the vehicle, leading to its wheels). The ratio of the speeds of the transmission output and input (the “transmission ratio”) is typically not the same as the variator ratio, being modified by the gearing.
Such gearing may include a “shunt” gear, typically of epicyclic type, enabling the transmission to provide very low, and even zero, output speed while its input is being driven. The shunt has a first part coupled to the variator's input side, a second part coupled to the variator's output side, and a third part coupled to the transmission output. By suitable design of the shunt gearing it can be arranged that at a certain variator ratio, the speeds of the first two parts cancel each other out, and the transmission output is thus stationary even though it remains mechanically coupled to the rotating transmission input. This condition in which the transmission provides an infinite speed reduction is referred to as “geared neutral”. Variator ratios on opposite sides of geared neutral provide opposite directions of rotation at the transmission output (forward and reverse, in a vehicle transmission).
A transmission of this type is in principle able to drive a motor vehicle at a sustained and very slow speed, a facility which is potentially very useful. Problems arise, however, in controlling the variator under such conditions.
Before explaining the problems, it is necessary to say something about the construction and control of a variator. A variator of toroidal-race rolling-traction type is illustrated in FIGS. 1 and 2. This type of variator is in itself well known in the art. It is presented here merely to illustrate certain relevant principles.
The illustrated variator 10 has twin toroidal cavities 12a, b, each defined between a respective input race 14a, b and output race 16a, b. The races are mounted for rotation about a common axis 15, defined in this example by a variator shaft 17. Facing surfaces of the input/output race pairs are semi-toroidally recessed (as seen at 18 in FIG. 2 and indicated by dashed lines in FIG. 1), and within each of the cavities 12a, b is a set of rollers 20a, b running on the recessed surfaces. The rollers serve to transfer drive between the input and output races. In the illustrated example each cavity 12a, b contains three rollers 20a, b, although FIG. 1 shows only one of those, for the sake of simplicity. Each roller has an axis, which in FIG. 1 is perpendicular to the plane of the paper and is indicated at 22a, b, and is mounted in a yoke 24a, b for rotation about its axis. The two output races 16a, b are coupled to rotate together. In the illustrated example this coupling is made through the shaft 17 upon which they are both mounted e.g. through splines (not seen). The two input races are also coupled to rotate together, e.g. through a sleeve 26, but are able to rotate independently of the shaft 17, being mounted upon it through bearings (not seen). The sleeve 26 in this example carries a pulley or gear 29 which engages with a bolt or chain (not shown) to form the variator output. A force is applied as indicated by an arrow 28 to urge the races 14a, b, 16a, b into engagement with the rollers and so provide roller/race traction.
When the shaft 17 and the input races 14a, b carried upon it are driven to rotate, they cause the rollers 20a, b to spin about their axes, and the rollers drive the output races 16a, b. In this way the variator transmits drive.
The rollers are able to move to steplessly vary the variator ratio. In the illustrated example the yoke 24a, b of each roller is connected through a piston rod 34a, b to a respective piston 36a, b running in a cylinder 38a, b to form a hydraulic actuator. As the piston 36a, b moves in its cylinder, its roller 20a, b moves along a circular path about common axis 15. Note also that each roller is able to undergo a tilting motion, turning about an inclined axis 39a, b defined by its coupling to the piston 36a, b. As each roller moves back and forth it suffers a steering effect due to the action of the races upon it, causing it automatically to tilt to find a position in which the roller axis 22a, b intersects the common axis 15 of the races. All of the rollers 20a, b undergo such motion substantially in unison. The roller's tilting motion causes a change in the relative speeds of the input and output races—i.e a change in variator speed ratio.
When power is transmitted through the variator, the races exert a net force on the rollers tending to move them along their circular path about the common axis 15. Since this force acts about the axis, at a distance from it equal to the radius of said circular path, it can be expressed as a torque (force multiplied by distance) acting about the axis. This torque must be reacted through the actuators 36, 38 to a fixed object such as the variator's casing (not shown). The sum of such torques acting on all of the rollers is the total torque reacted to the casing and is thus referred to as the “reaction torque”.
The force excited upon each roller by the races must be directly balanced by the force exerted on the roller by its actuator 36, 38. Hence by setting the actuator's force (which is determined by a difference in pressures between hydraulic lines 40a, b supplying opposite sides of each piston 36a, b), reaction torque is directly set.
Furthermore reaction torque must clearly be equal to the net torque applied to the variator—i.e. to the sum of the torques acting on its input and output. This quantity is thus directly controlled by controlling the actuator force.
Note that the physical system illustrated in FIGS. 1 and 2 provides no direct means of setting a required variator speed ratio. Instead, changes in ratio take place automatically, by virtue of the physical construction of the transmission, as a result of the torques at the variator's input and output. To appreciate this, consider the grossly simplified and schematic representation provided in FIG. 3. An engine exerts an engine torque TE on the transmission input side. The action of the variator creates a torque TIN on the same side of the transmission. Both act upon the inertia WIN referred to the variator input (contributed by rotating parts of the engine and transmission). The net torque TE+TIN acts on the inertia WIN and, if the net torque is non-zero, causes it to accelerate. Variator speed ratio automatically changes to accommodate such acceleration. On its output side of the transmission's torque TOUT is added to any torque TB from the vehicle brakes and torque TW at the wheels due to drag the vehicle being on a slope etc to provide a net torque acting on inertia WOUT (which includes the inertia of the vehicle itself, as well as that of the final drive etc) to determine its acceleration. Again, variator speed ratio changes automatically to accommodate the acceleration.
A variator arranged to regulate reaction torque is sometimes referred to in the literature as being “torque controlled” to distinguish from more conventional transmissions in which ratio is directly regulated, such transmission being referred to as “ratio controlled”.
Many known transmissions use hydro-mechanical feedback to achieve ratio control of the variator. For example, a valve may be provided to set the hydraulic pressure controlling the variator, the valve being itself controlled through (a) a mechanical connection to the one of the rollers, to sense its position and (b) a mechanical input signal representing the required ratio. The valve serves to compare the two signals and to modulate the piston pressure to achieve the required roller position. Such systems are typically implemented using variators somewhat different in construction from the one seen in FIGS. 1 and 2. An example can be found in US 2003/0228952 (Joe et al). The present invention is not concerned with transmissions of this type.
Successful torque controlled transmissions have to date typically been reliant on sophisticated control strategies implemented in software in which engine torque demand and variator reaction torque are controlled in a coordinated manner. Examples are to be found in published international patent application WO 04/085190 (application PCT/EP04/03293).
For the sake of completeness and clarity, FIG. 7 illustrates in a highly simplified form one example of a complete transmission including an epicyclic “shunt” gear arrangement. The motor vehicle's engine E drives the input shaft 17 of the variator 10. The variator shaft 17 thus forms the variator's rotary input, and also the input of the transmission as a whole. A gear train 700 also couples the variator shaft 17 to a shaft 702 of a planet carrier 704 of an epicyclic gear train 706. Sun gear 708 of the epicyclic gear train 706 is coupled by a chain drive 710 to the variator's output races 16a, b, which thus form the variator's rotary output. Ring gear 712 of the epicyclic gear train 706 is coupled to a rotor 714 forming the transmission output. In the drawing the rotor 714 is shown to be coupled directly to the vehicle wheels 716, although in reality this coupling is normally made via the driveshaft, differential gear etc.
Stability is a crucial factor with regard to both the physical design of the transmission and the method used to control it. The variator rollers would, in the absence of suitable damping, suffer unwanted oscillation. The most simple mode of oscillation involves movement of all of the rollers in unison about their equilibrium positions, with consequent variation of transmission ratio which can be experienced by vehicle passengers as judder or vibration.
The type of variator presented in FIGS. 1 and 2 is typically provided with hydraulic damping to overcome the problem. FIG. 4 shows a suitable hydraulic arrangement in highly schematic form. Pressure control valves P1 and P2 are supplied with pressurised hydraulic fluid by a pump (not shown). An electronic controller 50 sends to each of the valves a respective pressure demand, and in response the valves each output a corresponding pressure through associated supply lines S1 and S2 feeding opposite sides of the variator pistons 36 (only one of which is shown in FIG. 3). By controlling the valves P1 and P2 the electronic controller controls the piston force and hence the variator reaction torque. Note however that the supply lines S1 and S2 incorporate respective damping orifices O1 and O2. These are formed as constrictions in the supply lines, and typically as sharp-edged orifices, whose flow resistance does not vary greatly with fluid viscosity, and hence with temperature. Piston movement creates flow through the orifices, and that flow creates pressure changes across the orifices O1 and O2 tending to resist the piston movement. The orifices thus to give rise to a force which opposes piston motion and is related to piston speed. In the conventional analysis of harmonic motion such a term is regarded as providing damping, and that is its effect in the present context.
Note also that the hydraulics possess a degree of compliance. In FIG. 4 the compliance is represented by accumulators C1 and C2. In practice, a degree of compliance is created even without provision of accumulators as such by virtue of compressibility of the hydraulic fluid (which becomes somewhat aerated in use), the volumes of the working chambers on either side of the pistons, etc. The compliance creates a phase lag between fluid flow and the aforementioned pressure changes. This is again desirable with regard to variator stability. Production pressure control valves typically exhibit a lead term between flow and pressure. The illustrated hydraulic circuit prevents the variator and the valves interacting in an unstable way by cancelling the lead term from the valves by the lag term due to the compliance. Also the variator, even with the damping orifices, proves potentially unstable when coupled to a vehicle drive line, which can be thought of in the context as a torsion spring since it can store energy due to “wind up”. The inclusion of hydraulic compliance makes the variator behave like a rotational damper. It thus damps the driveline oscillations rather than inciting them.
A less desirable effect of the damping is that it produces a lag between changes in the pressures demanded of the valves P1 and P2, and corresponding changes in the pressures applied to the pistons 36. The effect is equivalent to applying a low pass filter to the pressure demand, as illustrated in FIG. 5. The dotted line represents a pressure demand, which is in this particular example a sine wave, merely for purposes of illustration. The continuous line represents the actual pressure applied to the piston 36, and can be seen to lag behind the demand by a lag time Tor.
In order to provide good transmission control at low speeds, the system controlling the transmission must be able to react quickly to changing conditions. The hydromechanical damping makes this problematic. Suppose for example that the electronic controller is programmed to apply closed loop control to regulate the transmission. The combination of (a) rapid and responsive closed loop control through the electronics with (b) a damped hydromechanical system is found to be potentially unstable due to the phase lag Tor, which can give rise to positive feedback. Addressing this problem is an object of the present invention.
Consider now some of the situations in which vehicles are required to maintain a low speed. Various such situations arise in relation to agricultural vehicles, e.g. tractors.
When ploughing, the load on the engine is contributed mainly by the plough itself and varies according to the nature of the soil. It is desirable to maintain a fairly brisk pace, but this must be done without allowing the engine to stall when the plough encounters unyielding soil. Maintenance of a constant ground speed is not of great importance. The soil behaves in this context somewhat like a viscous fluid, so that the force needed to propel the plough is roughly proportional to speed. If, with a fixed transmission speed ratio, the engine struggles and slows, the load is thus reduced and engine stall avoided.
When rotivating (treating the soil using a rotary implement driven from the tractor's power take off, which is driven from the engine through a fixed ratio transmission separate from the one driving the vehicle wheels) engine load is contributed largely by the rotivator, and may cause the engine to slow when unyielding soil is encountered. It is desirable that there should be a constant ratio of vehicle wheel speed to rotivator speed. This can be provided by operating at fixed transmission speed ratio.
It is sometimes necessary simply to maintain a very low ground speed. For example a tractor may be required to pass very slowly by fruit pickers loading fruit into a trailer. A water blasting arrangement, for cleaning etc., may need to be towed very slowly past a building being blasted. Required speeds can be as low as 30 meters per hour. The transmission speed ratio required to provide such low ground speed is so low that no significant load can be applied to the engine, so with a speed governed engine a constant transmission ratio gives constant speed.
In all of these examples, the required results can be obtained by controlling the transmission to provide a constant—and very low—transmission ratio. Management of a torque controlled variator to provide this is problematic, however. Again, it is important to emphasise the difference in this respect between a ratio controlled transmission and a torque controlled transmission. In the former, control electronics set the required ratio and the hydraulics (e.g. the valve mentioned above, responsive to roller position and variator ratio) automatically control the variator to adjust it to that ratio. In a torque controlled transmission, the hydraulics regulate reaction torque. If reaction torque were not adjusted, transmission ratio would vary as, for example, the vehicle encountered inclines or other obstacles, or the load from a plough changed as the plough was raised and lowered. To maintain constant ratio the electronics controlling the variator must be able to rapidly adjust the reaction torque demanded of the hydraulics.
A stern test of a transmission control system involves driving the vehicle at very low speed over an obstacle in the form, of a square beam of say 30 centimeters height and width, the aim being to maintain a constant speed as the vehicle wheels first climb the beam's front face, placing a load on the power train, then reach its upper face, abruptly unloading it, and then begin to descend the beam's rear face, requiring the engine to be abruptly placed in overrun (engine braking). The trial is straightforward for a tractor with a conventional (stepped ratio) gearbox, since the transmission ratio cannot fluctuate and the engine speed can be taken to be constant (due to the use of a speed governed diesel engine having its own speed controller). It is highly challenging for a tractor with a torque controlled CVT, in which reaction torque must be rapidly adjusted in accordance with the changes in wheel load in order to maintain the required ratio.